The present invention relates to a piezoelectric filter device having a plurality of piezoelectric resonators and designed for use with a portable telephone set or the like.
In general, a conventional piezoelectric filter device of this kind comprises one or more pairs of piezoelectric filters, a corresponding number of terminal plates for electro-mechanically holding respective resonators at oscillation nodes and insulating plates arranged in a metal casing and electrically connected to form a ladder-type filter circuit as illustrated in FIG. 1 or 2. The ladder-type filter circuit illustrated in FIG. 1 is of an inverted L connection type comprising a piezoelectric resonator A branched in series and a piezoelectric resonator B branched in parallel. The ladder-type filter circuit illustrated in FIG. 2 is of a multistage type comprising four piezoelectric resonators A for series branch and three piezoelectric resonators B for parallel branch.
Besides, connector legs extend from some of the terminal plates such as an input and output terminal plates through the opening of the casing, which is closed by an insulating filler made of a resin material such as epoxy resin. The portion of each connector leg extending externally through the insulating filler is cranked or bent so that the filter device may be surface-mounted on a printed-circuit board by way of the externally extended portions of the connector legs. An example of such a conventional filter device is disclosed in U.S. Pat. No. 4,864,259.
In the conventional filter device as mentioned above the opening of the metal casing into which the insulating filler is filled has a smooth inner surface. When the insulating filler inserted into the opening is solidified and bonded the filler may be contracted so as to produce some crevices between the inner surface of the opening and the filler. Consequently, the tightness of the metal casing may be impaired and thus condensation may form the inner portion of the metal casing thereby resulting in a poor insulation.
The connector legs are bonded to electrical conductive paths printed on the printed-circuit board by soldering paste. If the soldering paste is laid too thick on the surface of the conducting members, wax may flow out of the soldering paste to make the operation of mounting the piezoelectric filter unnecessarily expensive and the overall height of the piezoelectric filter mounted on the printed-circuit board undesirably high. There are disadvantages that in turn hinder any attempt at reducing the size of the portable telephone set in which the filter is incorporated. In order to overcome this problem, soldering paste is recently required to be laid on very thin to form a layer having thickness of approximately 100 .mu.m. Since such a thin layer of solder can adversely affect the strength of bonding, a method of using a very thin connector having a thickness of approximately 0.1 mm and pressing it on heated soldering paste has recently been introduced. With such a method, the soldering paste partly comes up and covers the upper surface of the connector to substantially increase the overall bonding area.
A piezoelectric filter prepared in this way is then automatically mounted on the printed-circuit board and rigidly fitted thereto by soldering. In this mounting process, generally, a chucker is used to hold the piezoelectric filter at the free end of the connector legs projecting from the metal casing and the rear wall of the casing and carries it to a mounting station using a chip mounter. However, since the connector legs are made very thin as described above, each connector leg held by the chucker can be easily deformed by the force exerted and lose its flatness at its free end which is connected to the printed-circuit board. Therefore, a piezoelectric filter having such deformed connector legs cannot be fitted to a printed-circuit board in a neat and orderly manner.
Also, the thinning of the connector legs eventually reduces the stiffness thereof and results in the unstable mounting of the filter device on the printed-circuit board.